A method for producing a lactone by hydrogenating a dicarboxylic acid, a dicarboxylic acid anhydride and/or a dicarboxylic acid ester, has been studied since long ago, and various catalysts have been found.
For example, many proposals have been made on a process for producing a lactone by a fixed bed or liquid phase or suspension phase hydrogenation reaction system by using e.g. a nickel-type catalyst (e.g. Japanese Examined Patent Publication No. 6947/1968), a cobalt-type catalyst (e.g. Japanese Unexamined Patent Publication No. 95057/1976), a copper-chromium-type catalyst (e.g. Japanese Examined Patent Publication No. 20119/1963) and a copper-zinc-type catalyst (e.g. Japanese Examined Patent Publication No. 14463/1967). On the other hand, it is also known to produce a lactone by conducting the above-mentioned hydrogenation reaction by using a ruthenium catalyst for a homogeneous system. For example, U.S. Pat. No. 3,957,827 discloses a hydrogenation reaction under a condition of from 40 to 400 psi by using a catalyst of [RuX.sub.n (PR.sub.1 R.sub.2 R.sub.3).sub.x L.sub.y ] type. U.S. Pat. No. 4,485,246 discloses that a hydrogenation reaction by means of a similar catalyst is conducted in the presence of an organic amine.
However, such conventional methods wherein the nickel-type catalyst, the cobalt-type catalyst, the copper-chromium-type catalyst and the copper-zinc-type catalyst was used, all had a problem that it was necessary to employ a severe condition of a few tens atm. or higher. On the other hand, the conventional method wherein a ruthenium catalyst for a homogeneous system was used, had not only a drawback that the catalytic activity was slightly low, but also fatal problems that the catalytically useful life was extremely short, and the reactor was likely to be corroded by the use of halogen, although the method has a feature that the hydrogenation reaction proceeds under a relatively mild condition.